longestUnivaluePath.java

longestUnivaluePath.java

package leetcode.longestUnivaluePath;

import ds.*;

/**
 * Created by siyao on 9/30/17.
 * https://leetcode.com/contest/leetcode-weekly-contest-52/problems/longest-univalue-path/
 * Solution:
 * This does not sound like an easy level question.
 * It is not easy to come up with the correct induction or sub-problem.
 * So the sub-problm should be, rooting at each node, find the following:
 * 1. the single side uni-value path starting at the root.
 * 2. the answer of the question of the sub-tree.
 * info1 is critical for induction.
 *
 * T(n) = 2 * T(n/2) + O(1) = O(n).
 *
 * Thinking:
 * I did not come up with the induction quickly. Think more about it.
 *
 */

class Solution {
    public int longestUnivaluePath(TreeNode root) {
        return recur(root)[1];
    }

    // [0] is single side path, [1] is sub-max
    private int[] recur(TreeNode root) {
        if (root == null) {
            return new int[]{0, 0};
        }
        int[] l = recur(root.left);
        int[] r = recur(root.right);
        int bothSide = 0;
        int singleSide = 0;
        int v = root.val;
        if (root.left != null && v == root.left.val) {
            bothSide += 1 + l[0];
            singleSide = l[0] + 1;
        }
        if (root.right != null && v == root.right.val) {
            bothSide += 1 + r[0];
            singleSide = Math.max(singleSide, r[0] + 1);
        }
        bothSide = Math.max(Math.max(bothSide, l[1]), r[1]);
        return new int[]{singleSide, bothSide};
    }
}
/*
687. Longest Univalue Path My SubmissionsBack to Contest
User Accepted: 309
User Tried: 545
Total Accepted: 310
Total Submissions: 1089
Difficulty: Easy
Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.
Note: The length of path between two nodes is represented by the number of edges between them.
Example 1:
Input:
              5
             / \
            4   5
           / \   \
          1   1   5
Output:
2
Example 2:
Input:
              1
             / \
            4   5
           / \   \
          4   4   5
Output:
2
Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.
*/